In electronic devices it is often useful to be able to determine when another object is in proximity to it. For example in devices with speakers the device can be operated such that when a user is in proximity to the device the speaker does not output potentially harmful sound pressure levels (SPL) capable of temporally or permanently damaging the users hearing. Furthermore some devices can be controlled by determining object proximity or ‘gestures’. For example a call can be initialized by the detection of a ‘make call’ gesture or the volume of playback increased or decreased by the detection of a ‘change volume’ gesture.
Typically proximity detection can be implemented by the use of photo-detector which detects a lowering of the light level as the user approaches the device and casts the device in a shadow region. However such photo-detection requires a camera or other photo-detector equipment and furthermore can be found to be unreliable and inaccurate dependent on the placing of the sensor and the sensitivity of the light detecting sensor. In some embodiments a light source such as a LED can be employed on the device and the light level reflected from the neighbouring object detected to provide a rough location and motion estimate of the neighbouring object. However these rely on the reflectivity of the neighbouring object to determine an approximate distance and can therefore produce inaccurate distances for objects with reflectivity differing from the expected values.
Audio based proximity sensing has also been proposed where a predetermined audio signal is emitted by one or more speakers mounted in the electronic device and the corresponding signal is detected when it arrives at the microphone, either directly from the speaker or reflected from objects in proximity to the device. The time taken for the audio signal to travel from the loudspeaker to the microphone is then measured and a distance from the reflecting object to the device that emits and detects the sound can thus be determined knowing the speed of sound.
Devices using only one sound source (speaker) and one sound sink (microphone) enables the time of flight estimation to provide information only about the distance of the object reflecting the audio signal, but cannot determine the location, the direction of the object, or the direction of motion of the object.
A sonar type of device configured to measure the time it takes for the audio burst to travel from the speaker to the microphone and determines the corresponding direction when the speed of sound is known in the surrounding medium, such as air or water using a microphone array consisting of at least two microphones can for example determine both distance and direction. Typically, the output of the array is the sum signal of all microphones. Turning the array and detecting the direction that provides the highest amount of energy of the signal of interest is the most straightforward method to estimating the direction of arrival.
This steering of the array, i.e. turning the array towards the point of interest without physically turning the device is typically implemented by using the sound wave interference phenomena resulting from adjusting microphone delay lines. For example, a two microphone array can be aligned off the perpendicular axis of the microphones by delaying the second microphone output signal relative to the first by certain amount before summing them up. The time delay providing the maximum energy of the sum signal can then be determined to correspond to the direction of arrival.
When the distance between the microphones, the required time delay, and the speed of sound are known, determining the direction of arrival of the sound source is possible by detecting the inter channel time and level differences and using simple trigonometry. A more straightforward method for estimating the direction of arrival is by detecting the amplitude differences of the microphone signals since the further the sound has to travel the more it is attenuated.
However such multiple microphone arrays implemented in devices increase the complexity of such devices and furthermore can require microphone elements to be placed at locations about the device further increasing cost and size of the device.